This invention relates to a single-chip solid-state color imaging device using a mosaic color filter array.
A color television pickup system operating in the field integration mode, and using a CCD (charge coupled device) or a MOS (metal oxide semiconductor) imaging device has been developed and practically realized. The solid-state imaging device includes a great number of photoelements (pixels) arranged in the horizontal and vertical directions. The field-integration-mode television pickup system is arranged to simultaneously read out signals of pixels on two adjacent horizontal lines for each field interval. This system has an advantage of producing no residual image of one field since signal charges of all the pixels are read out for every field interval.
In order to obtain a color image, a longitudinal striped color filter array or mosaic color filter array is disposed on the solid-state imaging device. In the imaging device for a color television system which requires high resolution, it is preferable to use the mosaic color filter array which is excellent in horizontal resolution. A great number of color filter elements are arranged in a certain pattern, with each element corresponding to a pixel.
A mosaic color filter array used in a field integration mode has unit filter arrays, each comprised of 2.times.4 filter elements. The respective unit filter array is comprised of an upper 2.times.2 filter array and a lower 2.times.2 filter array having the same combination of color filter elements. Four pairs of two adjacent color filter elements arranged in the vertical direction are different from each other in combination of colors. In the unit filter array, the spectral response characteristics of the color filter elements are so set that color electric signals obtained on the two adjacent horizontal lines become zero when an object of achromatic color is imaged.
A mosaic color filter array is known which is comprised of magenta (Mg), green (G), cyan (Cy) and yellow (Ye) color filter elements. In this connection reference is invited to a paper entitled "A Single Chip CCD Color Camera System Using Field Integration Mode" in "The Journal of the Institute of Television Engineers of Japan", Vol. 37, No. 10, pp. 855 to 862. In order that, in the color filter array having such color filter elements, a color signal Cn on an N line and color signal Cn+1 on an N+1 line may become zero when an object of achromatic color is imaged, the spectral response characteristics of the color filter elements are determined to allow electric signals represented by (Ye+Mg) and (Cy+G) in the upper 4-pixel array to be equal to each other in their signal quantity and electric signals represented by (Cy+Mg) and (Ye+G) in the lower 4-pixel array to be equal to each other in their signal quantity. The filter array has an excellent resolution.
When, however, a special object having no vertical correlation, such as horizontally extending stripes, is imaged by the solid-state imaging device using the mosaic color filter array as described above, there is a possibility that a false color signal will be generated. For an object whose black-to-white boundary extends in the horizontal direction, for example, if such a boundary is focused midway between the upper 4-pixel array and the lower 4-pixel array in the aforementioned unit filter array, then color signals obtained from the upper and lower 4-pixel arrays become zero, resulting in no false color signal. Where the black-to-white boundary is imaged at the middle of either the upper 4-pixel array or the lower 4-pixel array, color signal components are generated, since the color signals produced by the 4-pixel array with such a boundary imaged there never become zero. These color signals are false in nature since they should not be generated from the black-and-white object. It is to be noted that many complementary color filter elements are employed in the color filter array.
A mosaic color filter array formed of color filters of primary colors, that is, green (G), red (R) and blue (B) is known in this field of art, for which reference is invited to Japanese Laid-Open Patent Publication No. 60-125090. In the unit filter array employing the primary colors, the upper and lower 4-pixel arrays are formed of G, G, R and B filter elements. In this filter element array, even if a black-to-white boundary is focused at the middle of the 4-pixel array, a false signal, unlike the aforementioned filter array, is never produced. Furthermore, the aforementioned complementary-color filter array is superior to the primary-color filter array in resolution.
However, the latter filter array has a drawback as will be set forth below. A brightness signal Y is given by a baseband component of a horizontal scanning signal. That baseband component is given below, paying attention only to the number of filter elements used. EQU Y=1/2{(1/2)R+G+(1/2)B)}
From the relative spectral response characteristics of the R, G and B filter elements and the imaging device, a ratio of light energies passing through the filter elements and obtained from the imaging device is EQU R:G:B=1:0.9:0.5
Taking the spectral response characteristics into consideration, Y' is given below so that the brightness signal becomes unity when R=G=B=1 in electrical signals. ##EQU1## The brightness signal Ys of the standard color television system is EQU Ys=0.30R+0.59G+0.11B
That is, for the brightness signal obtained from the color filter array using the primary colors' filter elements, the green light component to which human eyes are most sensitive is smaller in magnitude than in the standard brightness signal and thus a green color would be reproduced somewhat darkly.